Linux (the Redhat flavor at least) generally keeps multiple versions of the kernel installed. This is done to maintain backwards stability and allow the selection of an older (tried and tested) kernel if the latest update fails in any way.
On a small system all these kernel versions can quickly add up and waste valuable space. In such case keeping only the needed versions (the latest and the one currently booted, if different) is an acceptable risk to take.
List installed kernels
One can easily list all installed kernel packages with:
yum list kernel
Yum underlines the kernel that’s currently running (and will not uninstall it).
Install the tools
To clean up we use a tool that’s not installed by default on most system but is available in the yum-utils package:
yum install yum-utils
Once this is done we can perform the cleanup. In this case we specify that we want to keep at most 2 different kernel releases:
package-cleanup --oldkernels --count=2
The limit of installed kernel packages can be made permanent by editing /etc/yum.conf to add (or modify if it already exists):